This invention relates to a hydraulic tool for use in a drill string, such as is used in the drilling of oil and gas wells. More particularly, it relates to a tool adapted to jar upwards or bump downwards for the purpose of freeing the drill string when it is stuck.
A drill string is normally comprised of several components. At the bottom of the string is the bit which, by way of illustration, may have an outside diameter of 8 3/4 inches. Above the bit is a series of tubular drill collars which provide drilling weight to the string. These collars might have an outside diameter of 6 3/4 inches. Finally, a series of joints of drill pipe extend from the drill collars to the drilling rig at ground surface. The drill pipe commonly has an outside diameter of 4 1/2 inches.
In some drilling operations, the drill string periodically becomes lodged in the well bore. As would be expected, it is usually the large diameter components which become stuck.
When this occurs, one of the techniques used to try to free the string involves delivering repeated upward jars or downward bumps to the stuck section. Under most circumstances, it is desirable to deliver impacts is only one direction with the tool. For example, in the case where the drill collars are pulled into a keyseat, one should only bump the collars downward to free them. To jar them upward would simply wedge them into the keyseat tighter than ever. On the other hand, when the bit gets trapped on bottom by sloughing shale, one only wishes to jar upwards.
In the past, separate tools have been used in the drilling string to effect bumping or jarring. These tools are usually positioned part way down in the string amongst the drill collars but above the stabilizers.
The first such tool, referred to as a bumper sub, comprises a tubular outer barrel slidably receiving a tubular inner mandrel. The mandrel connects at its upper end to the drill pipe and the barrel connects at its lower end to the drill collars. The mandrel carries a hammer element at its bottom end and the ends of the barrel are closed with suitable shoulders which limit the longitudinal movement of the mandrel. Mechanical locking means, such as a spline joint, is included in the tool to permit relative longitudinal movement of the mandrel and barrel but prevent relative rotational movement. To use the tool, the drill pipe is raised by the rig until the mandrel hammer contacts the upper shoulder of the barrel. The drill pipe is then dropped; the mandrel hammer smashes into the bottom shoulder of the barrel and transmits a terrific jolt to the drill collars beneath. This procedure is repeated as often as required. It is to be noted that the bumper sub operates on the principle of free fall of the drill pipe.
The second tool, termed a hydraulic jar, also comprises a telescoping inner mandrel and outer barrel. The mandrel is usually connected at its upper end to the drill pipe portion of the string. The barrel is connected at its lower end to the drill collar portion. The mandrel and barrel define an annular space between them. This space is closed at its ends by packing and a body of operating oil is retained within the space. The barrel wall is formed at its bottom end with a section of reduced interior diameter. The mandrel carries a sleeve-type valve which is positioned in the reduced diameter section when the tool is collapsed. When the drill pipe is raised, the mandrel valve is drawn upward through the reduced diameter section toward a large diameter of "free-stroke" chamber defined by the barrel wall. As this movement takes place, the oil trapped within the free-stroke chamber is compressed and pressurized to pressures in the order of 5,000 to 10,000 p.s.i. Oil leaks slowly from the free-stroke chamber to the reduced-diameter chamber through the narrow annular space defined between the valve and reduced-diameter section wall. The annular space is designed to permit the oil to leak through at a rate whereby the pressure within the free-stroke chamber is maintained at 5,000 to 10,000 p.s.i. This arrangement results in retardation of the relative longitudinal movement of the mandrel and barrel. The driller is able to use the rig to pull the drill pipe with a force of about 50,000 to 100,000 pounds over its weight (the free-stroke chamber usually has a cross sectional area in the order of 10 square inches so that a pressure therein of 5,000 to 10,000 p.s.i. provides a resistance in the order of 50,000 to 100,000 pounds). When this is done, the pipe stretches and in so doing stores a considerable amount of elastic energy. As the valve moves out of the reduced diameter section, the elastic energy stored in the drill pipe accelerates the mandrel upward and a hammer shoulder it carries slams into an anvil shoulder carried by the barrel and transmits an upward jolt to the stuck section.
The upward movement of the valve through the reduced diameter section and the free-stroke chamber to the point where the hammer and anvil contact is termed "the jarring stroke". The downward movement back through the free-stroke chamber and into the reduced diameter section is termed "the re-setting stroke".
It has been proposed in U.S. re-issue Pat. No. 23,356, issued to Storm, to provide a two-way hydraulic jar. Storm teaches positioning the restricted diameter section intermediate the ends of the barrel so that upper and lower free-stroke chambers of relatively short and equal length are defined at each end thereof. The mandrel and barrel are equipped with two sets of impact faces adapted to contact at the limits of the up and down movements of the mandrel. When the drill pipe is pulled up, the mandrel valve is drawn through the restricted section and the pipe is stretched; when the valve reaches the upper jar free-stroke chamber, the mandrel is jerked upwardly to its most extended position and delivers an upward jar. When the drill pipe weight is subsequently placed on the mandrel, the valve moves back down through the restricted section and the drill pipe is compressed; on the movement of the valve into the lower bumper free-stroke chamber, the compressed string elongates and the mandrel snaps downward to deliver a downward jar.
There are relatively few occasions, when dealing with stuck drill pipe, during which it is useful to alternatively jar up and down. What is desirable is to have a single tool which can jar repeatedly in either direction. The prior art tools do not work in this manner since weight must be put on the mandrel to force the valve down through the restricted section on the re-setting stroke. Since the weight compresses the lower portion of the string, there is an automatic, unwanted, downward jarring action. If one could accurately force the valve down just to the base of the restricted section and then start the next jarring stroke, this would not be a problem. However, the awkward equipment of a rig does not lend itself to accurate positioning of this type.